James Tate and Ann Jane Atcheson, please stand up

Everyone has dead ends in their tree.  This is one of mine that I have not pursued to any large extent, but it does vex me.  Here's what I have,

James Tate/Tait/Teit born about 1818 in Ireland married Ann Jane (or Jean) Atcheson born about 1817 also in Ireland.

James and Ann Jane had two children, George born in about 1848 and Sarah Jane born in 1850.  According to George Tate's naturalization record, he was born in Kingston, Ontario.  He further states he immigrated to the US in 1866.  Both George and Sarah Jane always state they were born in Canada.

I can find no trace of this family in Canada.  It's as though they do not exist.

The first time I find them is in the 1870 census.  In 1870, Sarah Jane Tate is already married to her husband, Nicholas White and they are living in Union City, Erie, PA.  They have one son, John, aged 1 year.  Her husband Nicholas White was also from Canada.  His family has been found in both the 1851 and 1861 censuses in Percy, Northumberland, Ontario.  The only oddity with them is that once they immigrated to the US, suddenly every child in the family claimed to be 3 years younger than they were in Canada.  Time travel??

Oddly, I find James, Ann Jane, and George Tate living in Township 4, Placer, California.  I really have little to no idea how they got out there, except that it appears they were working on the railroad.  Or at least James and George were (see below).

It appears they were only in California briefly though as I find them very close to their daughter Sarah Jane and her family in the 1880 census on their new farm in Bloomfield, Crawford, PA.  Their son George, still living with them.

Their lives in Pennsylvania did not last long unfortunately.  James Tate died on the 15th of Oct 1885 in Bloomfield, Crawford, PA.  Ann Jane died 30 May 1884, also in Bloomfield, Crawford, PA.

After their deaths, their son George Tate married and had two sons of his own.  His sister Sarah Jane died fairly young in 1906.  Just barely in time for her to have a Pennsylvania death certificate.  Sadly though if she hadn't made it to 1906 it's unlikely anyone would know her mother's surname.

Rumors swirling in our family were that the Tate's were not happy with Sarah Jane's marriage because Nicholas was Catholic (his mother was an O'Neill).  I have no proof one way or the other as to the Tate's religion, however based on the surnames (Tate and Atcheson) I think it's reasonable to assume they were Protestant.  Additionally while Sarah Jane, her husband and children were buried in the Catholic cemetery in town, her brother George was buried in what I would term the general cemetery.  Possibly it is Protestant but to my knowledge it is non-denominational.

So from 1870 until their deaths, I can trace James and Ann Jane fairly well.  But, where were they prior to 1870?  I cannot find them anywhere in Canada.  Obviously they either had to be in/near Percy, Canada for their daughter to meet Nicholas White, or possibly Sarah and Nicholas met in the US somewhere.

Questions:
1.) Why can't I find James and Ann Jane Tate in Canada anywhere?
2.) Why did they go to California with the RailRoad in 1870?
3.) It's really just 1 and 2, however I would like to know more about their lives ingeneral.